Plasma levels of high-density lipoprotein (HDL) cholesterol and its major HDL apolipoprotein, apoA-I, are inversely associated with atherosclerotic cardiovascular disease. Plasma HDL cholesterol and apoA-I levels are determined in part by the rate at which they are catabolized, but the mechanisms that modulate their catabolism in vivo remain incompletely understood. Both acute inflammatory states, such as sepsis, and chronic inflammatory states, such as the metabolic syndrome, are associated with low levels of HDL cholesterol and apoA-I, primarily due to increased catabolism. A major underlying hypothesis of this project has been that extracellular phospholipases are upregulated in response to inflammation and modulate systemic HDL metabolism through hydrolysis of HDL phospholipids. We cloned and characterized a new member of the lipase gene family that we termed endothelial lipase (EL). In the current cycle of this project, we have demonstrated that overexpression of EL in mice causes markedly reduced HDL due to increased catabolism, and antibody inhibition of EL in mice causes increased HDL due to reduced catabolism. In this competing renewal proposal, we will focus our efforts on EL toward achieving a greater understanding of its structure-function relationships, metabolic interactions with other genes that influence HDL, association with atherosclerosis, and its relationship to human physiology and pathophysiology. Specific Aim 1: To investigate the differences between EL and its highly homologous relatives LPL and HL with regard to the molecular basis of the lipid and lipoprotein preferences of EL compared with LPL and HL. Specific Aim 2: To test the hypothesis that in the liver, EL interacts with HL and SR-BI to influence HDL metabolism, selective uptake of cholesterol, and reverse cholesterol transport. Specific Aim 3: To determine the effects of chronic hepatic-specific EL expression on atherogenesis in mice. To compare these results with the effects of endothelial-specific EL expression on atherosclerosis. To test the hypotheses that endothelial EL expression influences endothelial physiology. Specific Aim 4: To test the hypothesis that EL is increased in inflammatory conditions in humans and may contribute to the reduced HDL-C levels associated with these conditions. A recent editorial stated: "Perhaps the most important questions concerning endothelial lipase are: a) does it play an important role in HDL metabolism in humans, and b) does it play a significant role in atherogenesis?" This proposal addresses both of these issues and also addresses key structure-function questions and potential gene-gene interactions with regard to EL and its effects on HDL metabolism. The results of these studies should provide important new insight into the role of EL in HDL metabolism and atherogenesis.